Symptoms
When start the FluentBooks application, you may not see the main window of the application. FluentBooks will be in the minimized state in the task bar.
Cause
This issue may happen by connect/disconnect monitor or changing screen resolution.
Solution
Remove the following file and try to restart FluentBooks
"%userprofile%\AppData\Roaming\FluentPro FluentBooks 2013\fluentpro.dmt.userdata"